When a graphene sheet formed by a chemical vapor deposition method (CVD method) or a thermal decomposition treatment method of a SiC substrate is used for an electronic device such as a transistor, a process of transferring graphene onto an insulating substrate from a growth substrate is needed. For example, in Patent Document 1, the following transfer process has been disclosed. On a graphene formed on a catalyst layer by a CVD method, a protective film of polymethyl methacrylate (PMMA), a photoresist, an electron-beam resist, and the like is formed, and then a support layer made of an acrylic resin, an epoxy resin, a thermal release tape, an adhesive tape, and the like is formed. Next, there is taken a method in which among the support layer, the protective film, the graphene film, and the catalyst layer, the catalyst layer is wet etched, and thereby the support layer, the protective film, and the graphene film are peeled off a growth substrate to be transferred onto another insulating substrate, and then the support layer and the protective film are removed. On the other hand, in Patent Documents 2 and 3, there is disclosed a manufacturing technique of a field-effect transistor using graphene for a channel layer of the transistor and using a high dielectric constant material (high-k material) for a gate insulating layer.
Patent Document 1: Japanese Laid-open Patent Publication No. 2011-105590
Patent Document 2: Japanese Laid-open Patent Publication No. 2011-175996
Patent Document 3: Japanese Laid-open Patent Publication No. 2011-114299
In a transfer process of graphene that has been widely used generally, as a protective film for graphene to be used at the time of transfer, various resists constituted by polymeric materials are used. However, there is a concern that the polymeric materials remain on the graphene as a residue even after a removal process, and when a polymeric residue is adsorbed to an edge portion and a local defect portion of the graphene in particular, removal of the residue is conceived to be very difficult. The polymeric residue causes nonessential carrier doping on the graphene. Therefore, in the graphene transistor using graphene for a channel, for example, deterioration of electrical conduction property and unstable operation are caused.
Further, for efficiently controlling carrier concentration of the graphene transistor, a larger gate capacitance is needed, so that a high dielectric constant material (high-k material) such as HfO2 is used as a gate insulating layer. However, it is difficult to directly form a thin film of the high-k material on graphene, and the gate insulating layer of the high-k material needs to be formed thickly in some degree. As the film thickness of the gate insulating layer increases, the gate capacitance decreases, so that thinning of the gate insulating layer of the high-k material is desired. Further, also from a viewpoint of low-voltage operation of the transistor, thinning of the gate insulating layer of the high-k material is requested.